This invention concerns the design of broadband feedback amplifiers, particularly such amplifiers having a frequency shaped gain characteristic.
The use of negative feedback, in which a portion of the output signal is combined out-of-phase with the input signal to reduce distortion generated in an amplifier, has been standard for many years. In the field of wire communications systems, however, transmission lines, be they open wire, twisted pairs of coaxial tubes, attenuate the transmitted signals as a function of the frequency content of the signals. As a result, in repeater amplifiers it is necessary to correct the distortion introduced by the transmission line as well as that generated within the amplifier. In addition, in a broadband communications network, where frequencies necessarily run into the VHF range, impedances must be matched between the amplifiers and the line to prevent signal reflections and standing waves.
Impedance matching may, of course, be accomplished by simply adding resistance in series with the line or in shunt with it. Such "brute force" methods, however, reduce the efficiency of the amplifier, and require more gain; furthermore, resistance added to the input greatly increases generated noise. Impedance matching is commonly accomplished through the use of a hybrid transformer coupling network in a feedback loop. In addition to introducing some signal loss and some noise degradation of the input, however, these networks tend to introduce considerable phase shift, particularly at high frequencies. This phase shift can produce stability problems in a high gain amplifier.
U.S. Pat. No. 2,541,322, which issued Feb. 13, 1951 to H. L. Barney, teaches the use of multiple feedback loops in an amplifier of one or more stages to provide simultaneously linear amplification and desired input and output impedances for matching purposes. Others have investigated and expanded on the solution of impedance matching in a broadband amplifier through the use of multiple loop feedback. See, for instance, U.S. Pat. No. 3,493,882, which issued to L. D. Seader et al., Feb. 3, 1970 and "Transistor Stage for Wideband Amplifiers" by G. B. B. Chaplin, C. J. N. Candy and A. J. Cole, Institution of Electrical Engineers -- Proceedings, Vol. 106, Part B, Supplement No. 16, May 1959, page 762, et seq. Each of these disclosures, however, describes a special case in which the feedback networks are resistive and the amplifier characteristic is linear; any gain shaping would require additional line build-out networks.
An object of this invention is impedance matching together with frequency dependent gain shaping in a broadband amplifier without the need for either hybrid coupling networks in a feedback loop or line build-out networks.
Another object is impedance matching together with frequency dependent gain shaping through the use of dual major loop feedback.
Still another object is the use of passive frequency dependent feedback networks to provide predetermined gain shaping and impedance matching in a broadband amplifier.